optabs.def (movmem_optab): Add movmem back for memmove().

2019-07-02  Aaron Sawdey  <acsawdey@linux.ibm.com>

	* optabs.def (movmem_optab): Add movmem back for memmove().
	* doc/md.texi: Add description of movmem pattern for overlapping move.

From-SVN: r272946